Thermoplastic foam products can be produced by a wide variety of processes, of which extrusion is but one, that are in part responsible for the wide variety of foam products available today. Foams range in consistency from rigid materials suitable for structural use to flexible substances for soft cushions and packaging materials. These foams range in cellular formation from open or interconnecting cell foams to closed or unicell foams. The cell structure may range from large to fine. Electrical, thermal, mechanical, and chemical properties can be varied within wide limits depending on the thermoplastic resin composition and the method chosen to create the foam. Foamed thermoplastics range in density anywhere from about 10 kg/m.sup.3 to over 1,000 kg/m.sup.3, although the latter perhaps more properly are called microcellular structures. True foams are considered to have a density of less than about 800 kg/m.sup.3.
Many methods have been developed for the manufacture of foamed thermoplastics, which generally can be classified into three groups: 1) methods for adding a gaseous "blowing agent" to the thermoplastic during processing, 2) methods for producing a gaseous blowing agent in the thermoplastic during processing, and 3) methods for forming a thermoplastic mass from granules to obtain a cellular structure. Similar blowing agents sometimes are used in the various methods to produce foams. However, the effectiveness of a particular blowing agent varies considerably depending on the thermoplastic resin composition, the method chosen, the process conditions, the additives used, and the product sought.
Blowing agents work by expanding a thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made. Bubbles of gas form around "nucleation sites" and are expanded by heat or reduced pressure or by a process of chemical reaction in which a gas is evolved. A nucleation site is a small particle or conglomerate of small particles that promotes the formation of a gas bubble in the resin. Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution.
However, the foam is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse. Additives may be incorporated into the resin and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
Methods for producing a blowing agent in situ usually involve a chemical reaction that evolves gas. Polyethylene, silicone, epoxy, and vinyl foams have all been produced by adding a substance that will produce a gaseous blowing agent chemically. For example, dinitroso compounds and hydrazides, which evolve nitrogen gas on decomposition, and bicarbonates, which evolve carbon dioxide, have been added to thermoplastic resins to produce foams.
Polystyrene foams often are produced by "bead molding," in which partially expanded granules or beads are heated in a mold in the presence of a blowing agent to expand and fuse the particles into a rigid unicellular structure. A volatile organic compound or some other gaseous blowing agent is impregnated into the beads. Heat is applied and the pressure is released to cause the beads to expand and fuse.
There are several methods for adding a blowing agent to a thermoplastic resin during processing to produce a foam. Ureaformaldehyde and polyvinylformaldehyde foams have been produced by whipping air into a heated thermoplastic mass before it sets. Polyolefinic foams have been produced by introducing air or some other gas or volatile solvent into a heated thermoplastic polyolefin mass and then heating the mass or reducing pressure to expand the gas and form pores of a desirable size. One more specific method is to impregnate a thermoplastic resin with blowing agent under heat and pressure in a closed vessel. The pressure is released to expand the blowing agent to form "prefoamed," or partially expanded, beads. Prefoamed beads usually are further expanded in an enclosed vessel such as a mold to produce a molded foam product.
Another more specific method, to which the invention claimed herein relates, is to mix the blowing agent with molten resin under pressure and then extrude the mixture through a forming die into a zone of reduced pressure. Shaped extruded foams can be produced by this method using a forming die of particular configuration. Plank, which can be cut to a desirable shape, and thin foam sheets are produced in this manner.
Many of the halogenated hydrocarbons have been used for several years as blowing agents in various methods for producing foams from thermoplastic resins. The halogenated hydrocarbons include the chlorofluorocarbons ("CFCs") and hydrochlorofluorocarbons ("HCFCs"). CFCs and HCFCs are readily impregnable in thermoplastic resins and are readily expandable under relatively mild conditions. CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty. The pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs, and other halogenated hydrocarbons typically are either not flammable or are of low flammability, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity. However, governmental regulation is phasing out use of many halogenated hydrocarbons because the halogenated hydrocarbons may be responsible for damage to the earth's ozone layer.
Producers of thermoplastic foam products have been seeking alternatives to CFC and HCFC blowing agents for a number of years to reduce or eliminate altogether the amount of halogenated hydrocarbons used. A number of volatile organic compounds (VOCs) have been proposed, although many of these also are somewhat objectionable. VOCs include the light aliphatic hydrocarbons such as propane, n-butane, isobutane, butylene, isobutene, pentane, neopentane, and hexane, to name but a few. The diffusivity of VOCs can be many times faster than that of the halogenated hydrocarbons and harder to control. Foam collapse and stability problems have been encountered, although high quality foams can be produced using VOCs. Also, VOCs typically are flammable, thus presenting handling problems and safety concerns.
Inert gases have also been proposed as blowing agents, although these sometimes do not provide acceptable results, especially for producing extruded foams. Inert gases include nitrogen, argon, xenon, krypton, helium, and carbon dioxide. Nitrogen and carbon dioxide, in particular, have the advantage of being inexpensive, readily available, and of not being flammable, and are not considered to be harmful to the earth's ozone layer. However, inert gases usually are not as readily soluble in thermoplastic resins as CFCs, HCFCs, and VOCs, are of higher volatility, and do not reduce the viscosity of the resin. Lubricants often are added to the resin for extrusion foaming. The range of processing conditions for producing acceptable products is narrower than for CFCs, HCFCs, and VOCs. Mixing the resin with an inert blowing agent and keeping the inert blowing agent in the resin is more complicated than for CFCs, HCFCs, and VOCs. The surface texture of extruded foams sometimes is rough. Extruded low density foams and thick foams are difficult to achieve. The foam sometimes has poor stability and foam shrinkage sometimes is uncontrollable.
Extrusion foaming is a continuous process in which a plasticized thermoplastic resin is cooled and expanded when the resin and blowing agent are extruded into a zone of lower pressure. Mixing of blowing agent with polyethylene resin for extrusion foaming can take place in as little as a few minutes or less because the resin is plasticized. Nucleating and stability control agents typically are used in extrusion foaming, even for CFC and HCFC blowing agents, to control cell formation, diffusivity of the blowing agent, and stability of the foam. No dispersion medium for the polymer particles is usually needed or used for extrusion foaming.
As an example of extrusion foaming, Watanabe et al. U.S. Pat. No. 4,214,054 disclose numerous volatile organic blowing agents including various CFCs, VOCs, and the use of decomposable gas-releasing chemical blowing agents for producing extruded polyolefin foams from particular resin compositions.
Nakamura et al. U.S. Pat. No. 4,649,001 propose preparing long, thin sheets of low density polyethylene foams by extruding a resin mixture of low density polyethylene and linear low density polyethylene. The extruded foam sheet is drawn at a rate faster than the extrusion rate to produce long, thin sheets. Foaming agents are said to include inorganic gases such as carbon dioxide and a number of VOCs and HCFCs. No specific blends of blowing agents are suggested.
Johnson U.S. Pat. No. 3,966,373 proposes a method and apparatus for making relatively dense structural foam profiles having a foam core and a dense skin. A partially expanded extruded thermoplastic polymer composition is conveyed through a chilled shaping passage moving at the same rate as the foam to eliminate friction. The dense skin is formed by the chilled passage while the polymer resin is still expanding. Foaming agents are said to include nitrogen, carbon dioxide, lower molecular weight paraffins such as propane, butane, and methylchloride, lower molecular weight olefins such as ethylene, propylene, and butylene or mixtures of the above. No specific mixtures of foaming agents are disclosed. A preferred thermoplastic composition for extrusion to form tongue depressors or ice cream sticks of 320 to 1000 kilograms per cubic meter is disclosed to include polystyrene beads having a pentane blowing agent integrated therewith.
An alternative blowing agent for extruded thermoplastic foams, a process for producing extruded foams using such a blowing agent, and a thermoplastic resin composition containing such a blowing agent would be desirable that avoid or substantially reduce the impact of the problems typically associated with CFCs, HCFCs, VOCs, and inert gases in producing extruded foams and are capable of producing foams having acceptable pore structure, density, and stability.